This simplest explanation is that JKR isn’t a very good writer. She came up with an idea “I’ll give hard working and ambitious Hermione a time travel device so that she can take more classes in the same amount of time” without thinking deeply about the implications of allowing time travel to exist in the universe of Harry Potter.
“But she wasn’t a good writer. She was mid tier at best. So the eventual success of the series got beyond her abilities. While the last book was much better overall than the first few, it still relied on shoddy world building”
Excellent explanation. The first HP book is excellent. It really sucks you in. After book 4, the quality declines and they become slogs to get through.